The present invention provides a process for reducing the metal ion content of a film-forming resin with good lithographic performance in light sensitive photoresist compositions, and for using such a film-forming resin in such light-sensitive compositions. The present invention also provides a process for preparing these film-forming resins by continuous. liquid/liquid extraction utilizing a liquid/liquid centrifuge and using such resins for making a high quality light-sensitive photoresist composition (including both positive and negative working photoresist compositions). Further, the present invention provides a process for coating a substrate with these light-sensitive compositions, as well as a process for forming an image and developing these light-sensitive mixtures on such a substrate.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components, such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of a film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked-coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed (in the case of positive photoresist) or the unexposed (in the case of negative photoresist) areas of the coated surface of the substrate.
Novolak resins are frequently used as the polymeric binder in positive liquid photoresist formulations. These resins are typically produced by running a condensation reaction between formaldehyde and one or more multi-substituted phenols, in the presence of an acid catalyst, such as oxalic acid, maleic acid, or maleic anhydride. In producing sophisticated semiconductor devices, it has become increasingly important to provide a film forming novolak resin of superior quality in terms of dissolution rate, better binding properties with a diazonaphthoquinone, and heat resistance. It is further imperative that these materials contain low metal contaminants since these impurities can adversely affect electrical circuitry in advanced microlithographic electronic devices. Metal levels of &lt;50 ppb or lower are commonly required for commercial resists sold on the market today. Although negative resist compositions are made from film-forming resins different from novolak resins, the same quality issues and requirement for low metal resins still apply.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. The etchant solution or plasma gases etch that portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive working photoresist compositions are currently favored over negative working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less than one micron is quite common. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.
In recent years there has been significant progress in novolak resin synthesis and methods of removing metal impurities. In a typical novolak reaction, a reactor is charged with phenolic compounds, an acid catalyst such as oxalic acid, maleic acid, p-toluene sulphonic acid or any mineral acid, and is heated to 95 to 100.degree. C. Formaldehyde is slowly added and the mixture is heated at reflux for 6 hours. At the end of the condensation period, the reactor is converted to distillation, and the temperature is raised to 200.degree. C. At this point vacuum is slowly drawn, the temperature is raised to 220.degree. C., and the pressure is reduced to below 20 mm Hg. After the volatiles have been distilled off, the vacuum is released and the molten resin collected and allowed to cool. In spite of using fairly pure starting materials and preventing contamination during synthesis, resin products often contain higher metal ion impurities than allowed for sale. Various metal ion removal processes have been described and are included herein for reference. In U.S. Pat. No. 5,378,802, K. Honda describes a method where a resist component in a solvent is treated with fibrous ion exchange resins that are subsequently removed by filtration. Szmanda and Carey teach a method of removing anions from organic solution by using a modified anion exchange resin having source anions less basic than hydroxyl anions. In a series of patents, U.S. Pat. No. 5,521,052, U.S. Pat. No. 5,543,263, U.S. Pat. No. 5,565,496, U.S. Pat. No. 5,594,098, U.S. Pat. No. 5,686,561, U.S. Pat. No. 5,858,627 and U.S. Pat. No. 5,955,570, Rahman et. al. describes metal ion reduction techniques utilizing sequential treatment with cationicm and anionic resins and the means of preparing these ion exchange media. This series of patents also teaches the advantages of utilizing ion exchange processes in polar solvents and the use of a specially constructed container filled with activated ion exchange resin. In addition, water washing with low conductivity de-ionized water has also been used to remove metals.
All of the aforementioned metal removal methods are time consuming and are essentially batch processes. Furthermore, introduction of acids or bases from the ion exchange media can affect photospeeds of the resists using resins prepared in this way. In some instances, changes in the resin (such as changes in molecular weight due to fractionation) are observed as a result of the carryover of small amounts of solubilized low molecular weight components of the resin into the aqueous phase of the ion-exchange media. The present invention overcomes these difficulties. The present invention also lends itself to a semicontinuous/continuous process and reduces the time needed to make low metals ion resins for photoresists. By using a semicontinuous or continuous liquid/liquid centrifuge where a resin solution can be introduced through one inlet port of the centrifuge while water or an aqueous solution of a metal ion chelating material is introduced through a second port, an efficient method of reducing metals ions in film-forming resins is achieved.